I've been playing with the some of features of beer tools pro . I see the affects
attenuation has on a recipe but I don't know what it is or how to achieve it or
higher numbers might do to the flavor of the beer. If someone could school
one this I would be great full .....Doug

Attenuation is basically a measure of how much sugar gets fermented to alcohol by the yeast. High attenuation will mean a lower final gravity and a drier, crisper beer. Lower attenuation will mean a sweeter, fuller bodied beer.

As far as controlling attenuation, there are yeasts that attenuate more or less than others, and grains (and extracts) that are less fermentable than others. Crystal Malts are a great example of grains that have a lower fermentability. Using a lot of Crystal Malt will leave a beer very full bodied, but also very sweet. Non-light extracts are usually lower attenuating than light extract, because they are generally made with grains that have lower fermentability.

There are also times when the yeast doesn't complete fermentation completely, leaving a beer underattenuated. This is basically the result of poor yeast health. It can be caused by factors like underpitching the correct yeast amount, not aerating the wort sufficiently before fermentation, fermenting too cold or warm, and reaching higher alcohol levels than the beer can stand.